The basis of the present invention is ultimately the problem of a reliable reverse flushing or cleaning of those strainers in a nuclear power plant that have the task of purifying water in the reactor containment before it is fed into a cooling system. In boiling water reactors the strainers are permanently under water at the bottom of the containment and convey water to a regularly working cooling system and/or to an emergency cooling system which is only started in case of a shutdown. Under normal circumstances, compressed water reactors lack water at the bottom of the containment. Here, the strainers shall only start working in connection with possible shutdowns, during which water is collected at the bottom of the containment. However, in both cases the strainers shall be capable of working in a reliable way in so far as reverse flushing or cleaning is initiated without delay, if and when the strainers are clogged by impurities to a certain extent. Hence, if the strainers are clogged to a too far-reaching extent, the flow of pure water to the cooling system is reduced to a level that may become dangerous. In practice, the degree of clogging of the strainers is controllable by measuring the pressure drop over the individual strainers. As long as the individual strainer is clean, i.e., free from clogging impurities, the pressure drop is minimal, but if fibres and/or other impurities begin to be deposited within and without the apertures of the strainer, the pressure drop increases gradually, to eventually reach a value at which the flow of water to the cooling system becomes inacceptably low.
In practice, the only realistic possibility of determining the pressure drop over a strainer is based upon a comparative measurement of gas pressures. More specifically, tests have been made to provide pressure indicators or sensors for each strainer, at the upper part of a couple of tube conduits, of which one at its lower part is laid to the interior of the strainer, while the other one has its lower part leading into the water mass surrounding the strainer. Thereby, the first sensor shall measure the pressure prevailing inside the strainer, while the second shall measure the reference pressure that prevails in the surrounding water. If the strainer is clean, i.e., it lets water through unobstructedly, these two measured pressures become equal, but as soon as the apertures of the strainers start to be clogged, the pressure within the strainer sinks. This manifests itself as a difference between the pressure that is measured by the two sensors, the magnitude of the pressure difference being a measure of the degree of clogging. Therefore, when the pressure difference has reached a given threshold value, reverse flushing or another cleaning shall be initiated. However, tests performed with measurements of pressure differences have not been successful due to difficulties in holding the tube conduits serving as reference legs clean, i.e., filled with gas only. Thus, if differently large water heads occur in the two reference legs, errors of measurement arise that make a reliable operation alarm or a steering of the strainer cleaning function impossible.
In a narrow aspect, the present invention aims at eliminating the above mentioned difficulties and create the prerequisites of a reliable measurement of pressure differences, preferably in order to make possible an adequate cleaning of strainers in nuclear power plants. In a wider aspect, the invention aims at providing a device which already in connection with elementary measuring of an absolute gas pressure functions in a satisfactory way. Thus, a primary object of the invention is to create a gas pressure measuring device having a tube conduit that is always kept dry and that is immersed in or immersable into a liquid.
In one aspect, the invention comprises a device for measuring gas pressure dependent on the level of a liquid surrounding a vessel. The device comprises a pressure indicator or sensor operatively connected via a conduit to a vessel, with volume within the vessel being greater than the volume within the conduit. Liquid entering into the vessel forms a surface whose level will vary depending on the level of the surrounding main liquid mass. Gas is retained within the conduit leading to the sensor as well as in any portion of the vessel above the liquid surface. The vessel includes a liquid inlet having a cross-sectional area smaller than the cross-sectional area of the vessel and located at or adjacent to the bottom of the vessel for entry of a liquid into the vessel whereby the liquid partly or substantially fills the vessel without rising into the conduit. The gas pressure within the conduit accordingly increases with an increase in level of the liquid within the vessel.
In a further aspect, the invention comprises two devices as characterized above, positioned substantially in a common horizontal plane. The inlets of the vessels of both devices open towards the surrounding main liquid mass with the inlet of the first device in direct communication with the liquid mass and the inlet of the second device being coupled to a strainer or filter for straining or filtering liquid passing into the second vessel. The filtered liquid may be further conveyed to a consumption unit such as cooling system in a nuclear plant. The first sensor establishes a base level, with the interior of the first vessel having a gas pressure substantially exclusively dependent on the liquid level of the main liquid mass. Within the second vessel the gas pressure is dependent on both the level of the main liquid mass and a pressure drop indicative of a clogging of the filter or strainer.